imwayland: Use serial to prevent redundant outbound changes

The serial was being used to silence away changes introduced by previous
commit calls. However the text_input protocol reads:

"When the client receives a done event with a serial different than the
number of past commit requests, it must proceed as normal, except it
should not change the current state of the zwp_text_input_v3 object."

It makes more sense to prevent the changes on the zwp_text_input object
itself, as those would be made redundant by more recent events. But the
effects of the .done calls should still take place.